[PATCH v8 1/2] PCI: Configure Root Port MPS during host probing

[Hans Zhang]

Current PCIe initialization logic may leave Root Ports operating with
non-optimal Maximum Payload Size (MPS) settings. The existing code in
pci_configure_mps() returns early for devices without an upstream bridge
which includes Root Ports, so their MPS values remain at firmware
defaults. This fails to utilize the controller's full capabilities,
leading to suboptimal data transfer efficiency across the PCIe hierarchy.

With this patch, during the host controller probing phase:
- When PCIe bus tuning is enabled (not PCIE_BUS_TUNE_OFF) and not
  PCIE_BUS_PEER2PEER (which requires the default 128 bytes for optimal
  peer-to-peer operation), and
- The device is a Root Port, the Root Port's MPS is set to its maximum
  supported value.

Note that this initial maximum MPS setting may be reduced later, during
downstream device enumeration, if any downstream device does not support
the Root Port's maximum MPS.

This change ensures Root Ports are initialized to their maximum MPS before
downstream devices negotiate MPS, while maintaining backward compatibility
via the PCIE_BUS_TUNE_OFF check and not interfering with the
PCIE_BUS_PEER2PEER strategy.

Suggested-by: Niklas Cassel <cassel at kernel.org>
Suggested-by: Manivannan Sadhasivam <mani at kernel.org>
Reviewed-by: Manivannan Sadhasivam <mani at kernel.org>
Signed-off-by: Hans Zhang <18255117159 at 163.com>
Tested-by: Mahesh Vaidya <mahesh.vaidya at altera.com>
Tested-by: Ricardo Pardini <ricardo at pardini.net> # on Odroid-HC4 
Tested-by: Shawn Lin <shawn.lin at rock-chips.com>
---
 drivers/pci/probe.c | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/drivers/pci/probe.c b/drivers/pci/probe.c
index 27008e2ea5af..dc4747858918 100644
--- a/drivers/pci/probe.c
+++ b/drivers/pci/probe.c
@@ -2229,6 +2229,18 @@ static void pci_configure_mps(struct pci_dev *dev)
 		return;
 	}
 
+	/*
+	 * Unless MPS strategy is PCIE_BUS_TUNE_OFF (don't touch MPS at all) or
+	 * PCIE_BUS_PEER2PEER (use minimum MPS for peer-to-peer), set Root Ports'
+	 * MPS to their maximum supported value. Depending on the MPS strategy
+	 * and MPSS of downstream devices, a Root Port's MPS may be reduced
+	 * later during device enumeration.
+	 */
+	if (pci_pcie_type(dev) == PCI_EXP_TYPE_ROOT_PORT &&
+	    pcie_bus_config != PCIE_BUS_TUNE_OFF &&
+	    pcie_bus_config != PCIE_BUS_PEER2PEER)
+		pcie_set_mps(dev, 128 << dev->pcie_mpss);
+
 	if (!bridge || !pci_is_pcie(bridge))
 		return;
 
-- 
2.34.1